Sarcoplasmic reticulum calcium ATPase. Labeling of a putative Mg2+ site by reaction with a carbodiimide and a spin-label.

The sarcoplasmic reticulum Ca(2+)-ATPase loses hydrolytic activity and the ability to be phosphorylated by Pi following incubation with EDC [1-ethyl-3-(3-dimethylaminopropyl)carbodiimide]. 4 nmol of tempamine per mg SR protein can be coupled to either a glu or an asp side chain through the EDC reaction. Mg2+ protects against loss of activity and tempamine labeling with a mid-point of about 3 mM in the absence of Ca2+. This is similar to the Kd for a Mg2+ that serves as a cofactor in enzyme phosphorylation. The Mg2+ protection constant is lowered by an order of magnitude when Ca2+ is bound to the transport sites. It is suggested that control of the Mg2+ binding site affinity may be part of the mechanism of enzyme activation by Ca2+.

Studies of cervical caps: I. Vaginal lesions associated with use of the Vimule cap.

Prior to investigating the contraceptive efficacy of cervical caps, we undertook a preliminary study to evaluate potential side effects of these devices. Women who had not previously used a cap were randomly assigned to wear either a Vimule or Cavity Rim Cap (CRC) for as long as seven days. The Vimule cap caused lesions of the portio vaginalis ranging from erythematous impressions to abrasions and frank lacerations. There was variation in the degree of trauma depending, in part, on the size of the cap and duration of wear. Disruption of the epithelium occurred in eight of twelve Vimule users, but the lesions were sometimes difficult to see owing to their location. CRCs were worn by 20 women. This device sometimes left a "suction ring" on the cervix but did not disrupt the epithelium. Two of three long-term users of the Vimule cap who were also studied had unusual formations of the vaginal mucosa suggesting a proliferative reaction to chronic irritation. It is recommended that all women using a Vimule Cap be carefully re-examined and counseled about further use of the device according to the findings of the examination.

PIP: Prior to investigating the contraceptive efficacy of cervical caps, we undertook a preliminary study to evaluate the potential side effects of these devices. Women who had not previously used a cap were randomly assigned to wear either a Vimule or Cavity Rim Cap (CRC) for as long as 7 days. The Vimule Cap caused lesions of the portio vaginalis ranging from erythematous impressions to abrasions and frank lacerations. There was variation in the degree of trauma depending, in part, on the size of the cap and duration of wear. Disruption of the epithelium occurred in 8 of 12 Vimule users, but the lesions were sometimes difficult to see owing to their location. CRCs were worn by 20 women. This device sometimes left a "suction ring" on the cervix but did not disrupt the epithelium. 2 of 3 longterm users of the Vimule cap who were also studied had unusual formations of the vaginal mucosa suggesting a proliferative reaction to chronic irritation. It is recommended that all women using a Vimule cap be carefully reexamined and counseled about further use of the device according to the examination findings.

Sensitivity of spin-labeled sarcoplasmic reticulum to the phosphorylation state of the catalytic site in aqueous media and in dimethyl sulfoxide.

Under controlled conditions, the iodoacetamide spin-label is highly selective for the sarcoplasmic reticulum adenosinetriphosphatase (SR ATPase), labeling 7-8 nmol of reactive residues per mg of SR protein, or approximately two residues per active enzyme unit. The electron paramagnetic resonance (EPR) spectrum of the labeled enzyme exhibits a small but specific broadening on the binding of substrates or inorganic phosphate. Addition of Ca2+ greatly increases the substrate broadening but reverses the effect of Pi. Here we demonstrate that the Ca2+ effect on the enzyme-substrate spectrum is due to the division of the major spectral component into two components, each representing two distinct conformational environments which impart slightly different degrees of mobility to the spin-label. These components are not readily separated in the spectrum of the fully labeled enzyme, and stoichiometric labeling techniques are used to resolve the components and obtain splitting parameters. The two spectral components exhibit random redistribution on removal and reintroduction of substrate, indicating that they may represent two forms of a given site. E-P formation by Pi in the absence of Ca2+ does not produce two discernible components in the EPR spectrum, although a small broadening effect is apparent, and two components can be resolved in the reaction kinetics of N-(1-oxy-2,2,6,6-tetramethyl-4-piperidinyl)iodoacetamide with SR. However, addition of dimethyl sulfoxide, which greatly facilitates phosphorylation of the enzyme, induces a two-component EPR spectrum. We conclude that a conformational change occurs in the enzyme specifically under conditions of high phosphate affinity which affects the motional parameters of 3-4 nmol of labeled residues per mg of SR, a number equal to the phosphorylation sites in our preparations. This affinity is induced by substrate and Ca2+ binding at activating sites in the normal mode of the enzymatic cycle, or by dimethyl sulfoxide in a general manner. In addition, we observe the conformational change with non-adenosine substrates in varying degrees, while 5'-adenylyl imidodiphosphate and adenosine 5'-(beta, gamma-methylenetriphosphate) are both fully effective. Since the latter analogues do not actively phosphorylate the enzyme, it is concluded that the conformational change is related to formation of a transition complex which is highly permissive of phosphoryl transfer. This complex is maximized for an adenosine moiety, while Ca2+ remains a stringent requirement.

Distribution of a fatty acid spin probe in sarcoplasmic reticulum. Evidence of membrane asymmetry.

The distribution of a lipophilic spin probe, 5-doxyl stearate, between the inner and outer halves of the sarcoplasmic reticulum (SR) bilayer was determined by titration with Ni X EDTA, a spin broadening agent. Titrations were also performed with Fe(CN)3-6 and with the solvated Ni2+ cation. Ni X EDTA titrations reached a clearly defined asymptote at 35% signal reduction. Fe(CN)3-6 and Ni2+ titrations gave biphasic curves but showed 35% of the signal to be readily eliminated at low concentrations. When the Ni2+ cation was used with ionophore, titrations indicated that 96% of the probe is aligned in the bilayer with the spin moiety at either the inner or outer interface. It was concluded that the spin probe distribution between the outer and inner halves of the SR bilayer is 35:65, respectively. Titrations performed on vesicles of purified SR lipids gave a ratio of 60 exposed:40 protected, consistent with the vesicular geometry. In addition the spin probe distribution in SR vesicles did not vary as a function of temperature, salt concentration, or spin probe concentration. On this basis it was concluded that the spin probe distribution gives a reasonable estimation of the volume of fluid lipids available to readily solubilize the probe in each half of the bilayer and that the observed asymmetry in distribution is due to the presence of SR proteins which were eliminated in the pure lipid vesicles. Furthermore, as EDTA is unique in its ability to chelate transition metals, Ca2+ and EGTA can be used in Ni X EDTA titrations without altering the chelation of Ni2+. Known changes in ATPase conformation accompanying Ca2+ and adenyl-5'-yl imidodiphosphate X Mg binding did not affect the spin probe distribution. However, phosphorylation of the enzyme by Pi gave a small, but clearly discernible, protection of spin probe signal. Chemical reduction with ascorbate indicated that this was due to occlusion of a small fraction of spin probes and thus possibly SR lipids.

Effect of diethyl pyrocarbonate modification on the calcium binding mechanism of the sarcoplasmic reticulum ATPase.

Diethyl pyrocarbonate was used to modify histidyl residues on the sarcoplasmic reticulum ATPase. Difference spectra of the N-carbethoxyhistidyl derivative indicated that most all the histidyl residues on the enzyme had been modified. These residues could be divided into two populations on the basis of their reaction rate with the reagent. It could then be shown that enzyme inhibition followed modification of the slower reacting population. Reversal with hydroxylamine verified that the loss of activity was due specifically to histidyl modification. Using [32P]ATP as a substrate it was further determined that the modified ATPase could form a phosphoenzyme intermediate, but that the hydrolysis of this intermediate was inhibited. Size exclusion chromatography was used to obtain equilibrium binding curves for high affinity Ca2+ sites on the enzyme. With the normal ATPase a cooperative binding curve for two Ca2+ with a Hill coefficient of 1.8 was observed. With the modified ATPase binding to two independent sites was observed; however, the dissociation constants remained the same as in the cooperative mechanism (K1 = 14 microM; K2 = 0.5 microM). That is, modification had eliminated cooperativity without changing the site specific binding affinities. E-P formation was then shown to follow binding to the higher affinity of the two sites. This would be the second site to bind Ca2+ in a sequential, cooperative mechanism. A model is suggested in which the binding of Ca2+ to an initial site allows for the binding of a second Ca2+ to an occluded site, this second site being responsible for enzyme activation. Modification apparently allows the binding properties of both sites to be observed independently.

Reactivity of sarcoplasmic reticulum adenosinetriphosphatase with iodoacetamide spin-label: evidence for two conformational states of the substrate binding sites.

The labeling kinetics of sarcoplasmic reticulum ATPase with the iodoacetamide spin probe N-(1-oxy-2,2,6,6-tetramethyl-4-piperidinyl)iodoacetamide were followed under conditions designed to selectively label all reactive groups. Approximately 1 mol of spin-label reacted per one 100 000-dalton ATPase chain, indicating only one residue on the enzyme had been labeled. One uniform rate of labeling was observed in the presence of Ca2+. When substrate was then added, approximately one-half of the residues showed a 10-fold increase in labeling rate while the remaining residues reacted at the initial, slower rate. Sequential labeling experiments further established that the two labeling rates correspond to the coexistence of two conformational state of the enzyme. Both Ca2+ and substrate are required to obtain an equal distribution between states, and the effect is completely reversed when substrate is removed. The iodoacetamide spin probe is known to be highly sensitive to the conformation of the ATPase binding pocket, and the residue labeled here is the one which generates broadening in the electron paramagnetic resonance spectrum on substrate binding. Due to the unique selectively of the labeling reaction, it is suggested that when both substrate and Ca2+ are bound to the enzyme, conditions which are precursory to enzyme phosphorylation, two specific conformations of the binding pocket exist in approximately at 50:50 ratio.

Ca2+-dependent effect of ATP on spin-labeled sarcoplasmic reticulum.

Vesicular fragments of sarcoplasmic reticulum (SR) were labeled with the --SH-directed spin label 2,2,6,6-tetra-methyl,4-amino(N-iodoacetamide). Colorimetric titrations of the remaining --SH residues and determinations of unbound spin label indicated that primarily 3 residues/enzyme molecule were labeled under saturating conditions. This labeling was accompanied by minimal losses in activity, providing precautions were taken to prevent sulfhydryl oxidation during the labeling process. Additions of ATP produced a new "highly constrained" component in the ESR spectrum of the labeled SR, an effect not noted in previous studies. It is demonstrated that the changes produced by ATP are reversible, and require both substrate binding and Ca2+ binding. However, hydrolysis of the substrate is not required. It is further demonstrated that the labeled residue(s) responsible for the spectral change is not in the immediate vicinity of the ATP binding site. It is apparent that the observed spectral change is related to a conformational effect of ATP and Ca2+ on the ATPase protein, which is associated with a large free energy change occurring on binding. It is also suggested that the conformational effect extends to a significant distance from the nucleotide binding site and may be a precursory step to Ca2+ translocation.

Oligovanadate binding to sarcoplasmic reticulum ATPase. Evidence for substrate analogue behavior.

Solutions of vanadate were controlled through concentration and pH adjustment to give specific compositions of mono- and oligovanadates. By monitoring the EPR spectrum of iodoacetamide spin-labeled ATPase, it is shown that decavanadate and the oligovanadate species present at neutral pH exhibit behavior typical of a substrate analogue. This is seen in terms of Ca2+ binding site affinity (microM), outward Ca2+ site orientation, and conformational effects on the enzyme normally associated with enzyme activation. In contrast, monovanadates exhibit behavior identical to that observed with Pi, with one exception: the vanadoenzyme is stable to Ca2+ in the concentration range of high affinity binding at the vanadate concentrations used here (200 microM). It is further demonstrated that Ca2+ binding in the 100 microM range directly induces enzyme devanadation of the monovanadate enzyme complex through Ca2+ binding to internal sites. Extensive array formation of dimeric ATPase units is found only with decavanadate in the absence of Ca2+, and then stoichiometric amounts are sufficient. Electron micrographs of dimeric arrays show evidence of increased penetration into the lipid bilayer, including freeze-fracture replicas which show evidence of corresponding "pits" in the inner leaflet of the bilayer. In turn, EPR spectra provide a means of following vanadate binding to the ATPase per se, as well as monitoring Ca2+-induced changes in the vanadoenzyme conformation, as only binding to specific sites on the enzyme affect the EPR spectrum.

Utilization of arylazido-ATP by sarcoplasmic reticulum ATPase in the absence of calcium.

The ATP analog arylazido-ATP 5'-triphosphate) (3'-O-(3-[N-(4-azido-2-nitrophenyl)amino]propionyl)adenosine 5'-triphosphate) was shown to phosphorylate the calcium-ATPase from sarcoplasmic reticulum in the absence of calcium. Levels of 0.6 nmol of phosphoenzyme/mg of protein were attained. Calcium either at micromolar or millimolar concentrations did not affect the level of phosphoenzyme. A non-Michaelian dependence of the hydrolytic activity as a function of analog concentration was obtained in the absence of calcium. Calcium addition did not modify either the analog concentration dependence for the activation of hydrolysis or the maximal rate of hydrolysis. In the presence of micromolar calcium, arylazido-ATP promoted calcium accumulation inside the vesicles, and a steady-state level of 100 nmol of calcium/mg of protein was maintained. ESR spectra of spin-labeled ATPase showed that addition of the analog in the absence of calcium caused a spectral change, and the resulting spectral parameters were different from those obtained for ATP under similar conditions. Calcium addition did not cause any further modification of the spectra, which was clearly distinct from the change when ATP was used. The partition coefficient of the analog from a water medium into an organic phase was found to be 1 order of magnitude higher than that of ATP. It is suggested that it might be the hydrophobic nature of the analog which makes it bypass the calcium requirement for utilization of the substrate by the ATPase.

Charge-transfer studies of the availability of aromatic side chains of proteins in guanidine hydrochloride.

The ability of aromatic tryptophyl and tyrosyl side-chain donors to form charge-transfer (CT) complexes with the acceptor 1-methyl-3-carbamidopyridinium chloride has been used to investigate the degree of exposure of these aromatic residues in denaturated proteins. The coplanar geometry of the CT complexes requires that virtually a full ring face of the donor be available for interaction with the acceptor, and the aromatic donor residues of lysozyme, trypsin, chymotrypsin, and the zymogens of the latter two enzymes do not appear to be wholly "exposed" in 6 M guanidine hydrochloride. Comparison of the CT proerties of the proteins with the corresponding properties of model complexes suggests that the incomplete exposure is due at least in part to statistical fluctuations in the continuously mobile, randomly coiled polypeptide chain which result in residues being alternately fully exposed and partly covered. Reduction and alkylation of the disulfide cross-links increase the apparent availability of the aromatic residues but the exposure is still less than that expected from a comparable mixture of tryptophan and tyrosine residues. Previous studies on the exposure of the aromatic residues of lysozyme and trypsin in aqueous salt solutions, when taken together with the present results, further suggest that there are two distinct kinds of surface environment possible on native proteins in solution. Some residues appear to be located in areas of the protein surface which are characterized by relatively fixed or stable local conformations, and have apparent CT association constants closely resembling these of comparable model complexes. Other residues may be located in a region where the protein conformation is flexible or continuously mobile, as evidenced by their smaller apparent association constants. It is probably significant that Trp-62 of lysozyme and Trp-215 of trypsin, both specificity site residues, appear to belong to the class of residues which can be considered as being in a flexible environment on the protein surface.

Two functional states of sarcoplasmic reticulum ATPase.

The "total" ATPase activity of rabbit sarcoplasmic reticulum (SR) vesicles includes a Ca2+-independent component ("basic") and Ca2+-dependent component ("extra"). Only the "extra" ATPase is coupled to Ca2+ transport. These activities can be measured under conditions in which the observed rates approximate maximal velocities. The "basic" ATPase is predominant in one of the various SR fractions obtained by prolonged density-gradient centrifugation of SR preparations already purified by repeated differential centrifugations and extractions at high ionic strength. This fraction (low dnesity, high cholesterol) has a protein composition nearly identical with that of other SR fractions in which the "extra" ATPase is predominant. In these other fractions the ratio of "extra" to "basic" ATPase activities is temperature dependent, being approximately 9.0 at 40 degrees C and 0.5 at 4 degrees C. In all the fractions and at all temperatures studied, similar steady-state levels of phosphorylated SR protein are obtained in the presence of ATP and Ca2+. Furthermore, in all cases the "basic" (Ca2+-independent) ATPase acquires total Ca2+ dependence upon addition of the nonionic detergent Triton X-100. This detergent also transforms the complex substrate dependence of the SRATPase into a simple dependence, displaying a single value for the apparent Km. The experimental findings indicate that the ATPase of rabbit SR exists in two distinct functional states (E1 and E2), only one of which (E2) is coupled to Ca2+ transport. The E1 in equilibrium E2 equilibrium is temperature-dependent and entropy-driven, indicative of its relation to the physical state of the ATPase protein in its membrane environment. Thenonlinearity of Arrhenius plots of Ca2+-dependent ("extra") ATPase activity and Ca2+ transport is explained in terms of simultaneous contribtuions from both the free energy of activation of enzyme catalysis and the free energy of conversion of E1 to E2. Thermal equilibrium between the two functional states is drastically altered by factors which affect membrane structure and local viscosity.

Flexibility in the specificity site of serine proteases.

The statistical availability of tryptophan and tyrosine residues with one ring face fully exposed to solvent was examined for two serine proteases and their derivatives by investigating the formation of charge transfer (CT) complexes between the aromatic donor residues of the protein and the acceptor 1-methylnicotinamide chloride. The availability of the ring face of one of the two exposed tryptophan residues in trypsin has been previously shown to be pH dependent and to parallel the acid side of the pH-activity profile of the enzyme. The present results indicate that, in diisopropylphosphoryl-trypsin (DIP-trypsin), this residue [which was identified as Trp-215 in native trypsin (chymotrypsin numbering)] is locked in a relatively rigid, pH-independent conformation with one ring face rotated out toward the solvent. In the zymogen and DIP-zymogen, the ring face is essentially unavailable. Chymotrypsin, like trypsin, has a pH-depent tryptophan residue available for complexation with the CT acceptor, but unlike trypsin, the pH dependence is apparently associated with dimerization of the enzyme. These and other data suggest this residue is the same as in the homologous trypsin structure, i.e., Trp 215, and that the ring face is mostly buried in the zymogen. Comparison of the crystal structure models of chymotrypsin and chymotrypsinogen shows that, as the specificity pocket opens up from its collapsed structure upon zymogen activation, the ring face of Trp-215 moves out and rotates relative to the surface of the enzyme in such a fashion as to become more accessible to solvent. These observations are in accord with the present CT results and provide additional support for the assignment of changes in Trp-215 availability to parallel changes in the conformation of the specificity pocket of these serine proteases. The present investigation also shows that, although a tryptophan ring face is partly exposed in DIP-chymotrypsin, its statistical availability more closely resembles that of the zymogen than the native enzyme. The reverse appears to be true for DIP-trypsin, which suggests the possibility that the specificity pocket in DIP-chymotrypsin may be partially collapsed while the catalytic residues are frozen in the conformation of the acyl-enzyme.

Interaction of CrATP with the phosphorylation site of the sarcoplasmic reticulum ATPase.

The chromium moiety of gamma,beta-bidentate CrATP slowly accepts a ligand from the sarcoplasmic reticulum Ca-ATPase to form an exchange inert coordination complex (k + 1 = 0.083 min-1; k - 2 = 0.003 min-1, 37 degrees C, 100 microM CaCl2). The stability of the Cr3+ coordinate bonds allowed the complex to be isolated by filtration techniques at neutral pH without acid precipitation. We found 4-5 nmol of [gamma-32P]CrATP to bind to 1 mg of sarcoplasmic reticulum protein with the subsequent occlusion of 7-8 nmol of 45Ca2+. At 37 degrees C, the CrATP.ATPase complex could be formed in the absence of Ca2+, although the reaction was 2-3 times slower than in the presence of Ca2+. Inhibition by Pi, by orthovanadate, and by fluorescein 5'-isothiocyanate verified that the bound CrATP was at the catalytic site. The site of CrATP attachment was found to be on the A tryptic fragment, possibly on the A2 subfragment. It was determined that Ca2+ binding to high affinity sites on the enzyme controls the rate by which the Cr3+ moiety accepts the ligand from the enzyme. The rate of change in the EPR spectrum of iodoacetamide spin-labeled ATPase was shown to follow the rate of ligand acceptance, rather than the binding of Ca2+ and substrate per se. This particular change has been attributed to the formation of an activated complex that is immediately precursory to phosphorylation and indicates here that this complex cannot be properly formed until the metal has been chelated by the enzyme. It is concluded that control over metal chelation (Cr3+ here, Mg2+ in the normal mechanism) is one means by which Ca2+ activates the enzyme.

Ca2+ binding to occluded sites in the CrATP-ATPase complex of sarcoplasmic reticulum: evidence for two independent high-affinity sites.

Occluded Ca2+ sites in the CrATP-ATPase complex are studied by first forming the complex in the presence of EGTA so that the sites can be occluded while vacant. 45Ca2+ binding to the occluded sites is then studied under equilibrium conditions. Binding curves are produced for two independent Ca2+ sites with Kd(1) = 0.2 microM and Kd(2) = 1.6 microM. When both sites are saturated, only the Ca2+ bound to the lower affinity site can exchange with free Ca2+. On addition of EGTA (15 vs 0.5 mM Ca2+) all bound Ca2+ dissociates, the net dissociation rate of one-half of the Ca2+ being approximately 10-fold greater than that of the other one-half (at 37 degrees C). When Ca2+ is bound only to the higher affinity site, this Ca2+ will exchange slowly if the concentration of free Ca2+ is below the saturation level of the lower affinity site. An ionophore dependency of the rates of binding and dissociation indicates that the access to the sites is through the interior of the vesicle. Solubilization in C12E9 releases the Ca2+ in the higher affinity site. Our observations are consistent with a model of the ATPase where the lower affinity of two transport sites is associated with the interior position (closest to the lumen) in a transmembrane channel. It is further evident that when in the occluded state, the higher affinity site is available without Ca2+ first being bound to the lower affinity site, eliminating cooperativity from the binding mechanism. In turn, this implies a connection between the integrity of the high-affinity binding site and the linking of sections of the catalytic site by CrATP.

Interaction of spin-labeled nucleotides with sarcoplasmic reticulum adenosinetriphosphatase.

Spin-labeled derivatives of AMP-PCP, ATP, and 2'-deoxy-ATP, with a nitroxide moiety attached to the ribose ring [3'-O-(1-oxy-2,2,5,5-tetramethylpyrroline-3-carbonyl)nucleotide], are used to study the nucleotide binding site stoichiometry of sarcoplasmic reticulum (SR) ATPase. With all derivatives, a maximal binding of 4.5 nmol/mg of SR protein is found, a value close to the number of phosphorylation sites obtained with ATP. The spin-labeled nucleotides cannot be utilized by the enzyme as substrates. Binding of spin-labeled nucleotides is inhibited by labeling the ATPase with fluorescein 5'-isothiocyanate, indicating that all the labeled nucleotide is located at the catalytic site. Additions of spin-labeled ATP to vesicle suspensions during steady turnover demonstrate competitive inhibition of both catalysis and the regulatory effect normally exhibited by ATP. As secondary binding of spin-labeled ATP is not detected at pertinent concentrations, it is suggested that both functions of ATP may be effected through a single site.

An iodoacetamide spin-label selectively labels a cysteine side chain in an occluded site on the sarcoplasmic reticulum Ca(2+)-ATPase.

Sarcoplasmic reticulum vesicles were labeled with [14C]iodoacetamide spin-label (ISL) under conditions where time courses of the reaction predicted that one amino acid residue would be preferentially labeled. Solubilized tryptic peptides were separated by high-performance liquid chromatography following extensive digestion, and amino acid sequences were determined for major and minor radio-labeled peptides. Only one radio-labeled residue, Cys-674 on the Ca(2+)-ATPase, could be identified. Extensive incubation with excess label increased nonspecific labeling, but did not produce detectable amounts of any other reactive side chain residue. Time courses of the iodoacetamide spin-label reaction were compared to those of 6-(iodoacetamido)fluorescein (IAF), and the ISL reaction was found to be more selective, in accordance with previous studies showing that IAF labeled both Cys-674 and Cys-670 [Bishop, J. E., Squire, T. C., Bigelow, D. J., & Inesi, G. (1988) Biochemistry 27, 5233-5240]. Titrations with spin-broadening reagents NiCl2 and Ni-EDTA showed Cys-674 to be in a region with very low solvent accessibility. These titrations also showed the ATPase to be distributed between two alternating conformations based on the accessibility of the label to NiCl2.

Nucleotide-binding sites in the functional unit of sarcoplasmic reticulum Ca2+-ATPase as studied by photoaffinity spin-labeled 2-N3-SL-ATP.

2-N3-SL-ATP [2-azido-2',3'-O-(1-oxyl-2,2,5,5-tetramethyl-3-carbonyl-pyrroline) adenosine triphosphate], a photoaffinity spin-labeled derivative of ATP with a nitroxide moiety attached to the ribose ring and an azido group attached to C2 of the adenine ring, was used to study the nucleotide-binding site stoichiometry of sarcoplasmic reticulum (SR) Ca2+-ATPase. The label was shown to bind at the catalytic site of the enzyme, even though the rate of hydrolysis was poor. A maximal binding ratio of 1 mol/mol of ATPase was found. The ESR spectra showed signals from spin-spin interactions between two radicals corresponding to a distance of about 15 A between labels bound to adjacent sites on the enzyme. This indicates that the minimal functional unit of the Ca2+-ATPase is a dimer with the nucleotide-binding sites in close proximity.